The invention relates to an electroslag remelting apparatus for remelting consumable electrodes to form ingots, said apparatus consisting of an ingot mold with a mold floor and a mold current connection, and a vertically movable electrode holder and feed device with an electrode current connection, the current connections being designed at least partly as straight busbars and forming an essentially coaxial or quasi-coaxial conducting arrangement with respect to the consumable electrode and ingot mold.
In electroslag remelting the melting power required to melt the electrode material is generated by the heating effect of the current in the slag bath. When using a single electrode the current flows from the electrode through the slag bath to the ingot. On account of the relatively high resistivity of the molten slag the desired current heating effect is mainly generated by the passage of the current through the slag. Electrical losses in the electrode and ingot are slight compared with the electrical power in the slag bath. Since a current can flow only in a closed electrically conducting circuit, a conducting loop containing the necessary power source must be present outside the electrode-slag-ingot arrangement.
Electroslag remelting is generally carried out with alternating current in order to eliminate the electrolytic effect of direct current. When operating an electroslag remelting apparatus with alternating current it must be ensured that the circuit does not have an inductive effect and that the electromagnetic stray fields are unable to heat the mechanical furnace parts. This effect becomes increasingly troublesome the greater the size of the apparatus since the currents that have to be used for larger ingot cross-sections are correspondingly higher. In order to eliminate this effect as far as possible, large-scale apparatuses are often operated with alternating current whose frequency is substantially below the mains frequency and is for example between 2 and 5 Hz. At such low frequencies the heating effect on the structural parts is very slight and in addition the inductivity of the current loop does not necessitate any additional measures in the largest apparatuses constructed hitherto. The current return conductors can be positioned far from the melting site so that no large currents can arise in the melt region between the current conductors. Such a melt power supply has the disadvantage however that costly frequency converters have to be employed.
To the extent that large ingots and large remelting apparatuses are also operated using mains frequency, the aforementioned conditions must be observed. It is for example absolutely essential to keep the inductivity of the whole furnace arrangement as low as possible since otherwise the bank of capacitors required to compensate for the reactive power (susceptance) would cancel out the cost advantages of the simpler power supply system. If the current return conductor is led very close along one side of the electrode, ingot mold and ingot, the transverse magnetic field due to the return current along the furnace axis is increased compared with the transverse field in an arrangement having a distant return conductor. This magnetic field has a disruptive effect on the electrode on account of the Lorenz force, which is dissipated by the return conductor. There may also be additional volume forces and flows in the slag bath. These physical effects can be avoided by a fully coaxial power supply arrangement, as is already generally known from high frequency techniques. The return conductor must enclose the electrode-slag-ingot arrangement as a tubular conductor. As regards the technical requirements, it is however sufficient to subdivide this tubular conductor into several, radially symmetrically divided individual current conductors. In the limiting case even two radially symmetrically arranged individual current conductors can still be regarded as a quasi-coaxial arrangement. Such an arrangement is sufficient for example to eliminate the disruptive forces acting on the electrode. The effect on the slag bath is also negligibly small with this same arrangement. In this connection there is also the beneficial effect resulting from the fact that the thick-walled ingot mold, which is generally made of copper, provides a lateral shield against the alternating magnetic field.
An electroslag remelting apparatus of the type described at the beginning is known for example from U.S. Pat. No. 3,684,001. In this apparatus the current return conductor is however in the form of a plurality of current ducts, i.e. is led quasi-coaxially, starting from the ingot mold floor via the upper mold edge to a swivellable furnace upper part on which the electrode feed device is mounted by means of an electrode holding rod, which is surrounded by an electrical sliding contact. The bifilar power supply terminates at the upper side of the furnace upper part and then passes into the afore-described coaxial system. In order to enable the furnace upper part to swivel it is necessary to provide releasable electrical couplings in the quasi-coaxial return conductors.
The known arrangement is consequently extremely tall, and its electrode length is roughly 1.5 times greater than that of the subject of the invention described hereinafter since in fact on account of the necessary construction principle the length of the electrode holding rod together with the pressure drive means arranged parallel thereto correspondingly increases the structural height. The arrangement of the return conductors also hinders access to the melt site. Replacement of electrodes is made more difficult and lateral movement of the ingot mold, for example onto a mold carriage, is quite impossible. The use of a sliding mold to produce long ingots in the manner of continuous casting leads to further structural problems, and the alternative use of a lowerable ingot mold floor necessitates additional sliding contacts; above all, replacement of an electrode is made so difficult that, using the known apparatus, it is a complicated and intricate matter to build up an ingot from a plurality of short consumable electrodes which are melted directly one after the other in the same ingot mold using a suitable electrode exchange mechanism. This disadvantage necessitates the use of extremely long consumable electodes, and accordingly the height of the apparatus is further unfortunately increased. The structural height of a remelting apparatus is always governed by the fact that either a correspondingly high remelting hall must be made available, or correspondingly expensive foundation work has to be carried out if the lower part of the remelting apparatus has to be arranged for example in a pit or depression.
The object of the invention is thus to provide an electroslag remelting apparatus of the type described at the beginning whose current paths run substantially coaxially and thus do not result in any interfering losses and damaging heating effects on structural parts, even when using mains frequency, and which at the same time guarantees good accessibility to the melt site for rapid replacement of the electrode and/or ingot mold, and which also has as small a structural height as possible.